Ecology, Concept and Theories in area Ecologists have documented striking patterns in the number of species occurring in different regions of the planet For example, for most groups of species, there is a strong gradient of declining diversity from the equator to the poles, and the vast majority of species are therefore concentrated in tropical and subtropical regions Ecologists have proposed a variety of mechanisms that may be responsible for generating global patterns of biodiversity For instance, areas with more varied landscapes and more topographic relief tend to support more diverse species assemblages than more uniform areas Thus, spatial heterogeneity is likely to provide more ecological niches for species to occupy and therefore allow more species to coexist As discussed earlier, classic ecological theory predicts that competitive interactions among species should lead to competitive exclusion and therefore reduced biodiversity Several of the proposed mechanisms for global diversity patterns focus on forces that counter the process of competitive exclusion Accordingly, it has been hypothesized that tropical regions harbor many species because their benign climate favors diseases and predatorsFthese predators and diseases increase species diversity by attacking competitively superior species, thereby allowing weak competitors an escape from competitive exclusion Disturbance is thought to similarly preclude competitive exclusion, although disturbance that is too intense or that occurs too frequently may result in reduced biodiversity The idea that diversity should be reduced when disturbance is either too rare or too frequent has been termed the intermediate disturbance hypothesis Another robust pattern of biodiversity is the observation that islands generally have fewer species than mainlands, and that larger islands tend to have more species than smaller islands R H MacArthur and E O Wilson proposed the equilibrium theory of island biogeography to account for this pattern According to this theory, there is a balance between colonizations of new species on islands and the subsequent extinctions of established species On larger islands, colonizations are relatively more frequent and extinctions are less frequent, resulting in a higher equilibrium number of species on larger islands Human activities such as the direct harvesting of species, introductions of alien species, habitat destruction, and various forms of habitat degradation have greatly accelerated the loss of biodiversity Consequently, current extinction rates are estimated be 100 to 1000 times higher than pre-human extinction rates This extinction crisis has spurred a great deal of scientific interest in the biological and ecological functions of biodiversity Organisms are responsible for a variety of ecosystem functions, including maintenance of the gaseous composition of the atmosphere, regulation of the global climate, generation and maintenance of soils, and recycling of nutrients and waste products However, it is not obvious whether these biological services require many species or only a handful of species Much current research is focused on understanding how much (if any) diversity we can afford to lose yet still maintain the necessary ecosystem functions (see Ecosystem Theories) Some species play obviously important roles in ecosystemsFthe addition or deletion of these ‘‘keystone’’ species leads to dramatic changes in ecosystem functions such as productivity or nutrient uptake However, most species probably not exert such important effects on ecosystems In other words, it may be possible to lose a number of species from an ecosystem with little overall impact on ecosystem function This could be the case if several species that perform approximately the same function are present in the original ecosystem The situation where multiple species play a similar role has been termed ‘‘species redundancy.’’ If species redundancy is a common phenomenon, ecosystem function should be largely independent of species diversity, so long as major functional types are represented Thus, when one species is lost from an ecosystem, some other species with a similar function may become abundant and compensate for the lost species, leaving the ecosystem as a whole relatively unaffected Indeed, ecosystem processes often remain stable despite large fluctuations in the abundance of the various species involved The term species redundancy may seem to imply that all species are not necessary for an ecosystem to function properly However, species redundancy may in fact be an essential feature for the long-term health of ecosystems, and the importance of any particular species may only become evident during the occurrence of rare, drastic events Spatial Patterns Spatial patterns and habitat patchiness or heterogeneity have spawned a host of concepts that try to both describe these spatial patterns and explain their impacts and sources The two most important general results of spatial theory in ecology are: The fact that species interact in spatially extensive and patchy worlds creates opportunities for persistence and coexistence of species that would otherwise not exist In uniform environments, the interplay of dispersal and nonlinear population dynamics can produce complex spatial patterning in population densities, and this patterning can itself promote coexistence in predator–prey or competitive interactions More generally, spatial theory emphasizes that it may be impossible to understand ecological processes and interactions if the focus and scale of a study is too small This is an important caution, given the fact that many experimental ecology studies are conducted at scales of less than m2 One of the most important practical applications of spatial theory stems from the use of metapopulation models in conservation biology Metapopulation theory describes a species as a collection of populations (or patches) in which there is turnover due to local extinction and subsequent recolonization by dispersing individuals, and in which the fate of a species can only be understood by tracking the fate of a collection of populations or patches, as opposed to a single local population or patch Metapopulation models are of considerable importance in conservation applications because they identify critical thresholds for habitat destruction or for the creation of barriers to plant and animal dispersal, thresholds that, once crossed, imply doom for a species (because colonization becomes too infrequent to counterbalance local extinctions) It is obvious to anyone who has looked out an airplane window that one of humankind’s major impacts